Prior to setting forth a short discussion of the related art, it may be helpful to set forth definitions of certain terms that will be used hereinafter.
The term “MIMO” as used herein, is defined as the use of multiple antennas at both the transmitter and receiver to improve communication performance, where more than one radio, supported by digital signal processing are used. MIMO offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. It achieves this goal by spreading the transmit power over or collecting the received signal from the antennas to achieve spatial multiplexing that improves the spectral efficiency (more bits per second per Hz of bandwidth) or to achieve a diversity gain that improves the link reliability (reduced fading), or increased antenna directivity.
The term “beamforming” sometimes referred to as “spatial filtering” as used herein, is a signal processing technique used in antenna arrays for directional signal transmission or reception. This is achieved by combining elements in the array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity.
The term “beamformer” as used herein refers to RF circuitry that implements transmit or receive beamforming and usually includes a splitter/combiner and may further include switches, controllable phase shifters, and in some cases controllable amplifiers and/or controllable attenuators.
The term “Transmit Radio Distribution Network” or “Tx RDN” or simply “RDN” as used herein is defined as a group of beamformers as set forth above.
The term “hybrid MIMO RDN” as used herein is defined as a MIMO system that employ two or more antennas per channel (N is the number of channels and M is the total number of antennas and M>N). This architecture employs a beamformer for each channel so that the radio circuit is connected to a splitter going to two or more antennas where phase and amplitude of each antenna signal could be adjusted individually.
In hybrid MIMO RDN transmitting systems, when the phases of the transmitted signals from each antenna are properly adjusted with respect to one another, the individual signals may be combined and result in an improved SINR for the receiving system.
FIG. 1 is a block diagram illustrating a standard 3G-4G cellular communication system 10 having a base station (BTS) 20 equipped with P antennas, 22-1 to 22-P. Additionally, a user equipment (UE) 11 in a MIMO configuration is shown. UE 11 includes an N channels MIMO baseband module 12 connected to N receivers 14-1 to 14-N and N transmitters 16-1 to 16-N in a duplex configuration which couples receivers and transmitters of the same channel to a corresponding antenna 18-1 to 18-N.
In operation, and in accordance with known 3G-4G standards, UE 11 transmits via antennas 18-1 to 18-N to base station 20 over an uplink channel (UL) both data and pilot signals. The signal properties of the pilot signals are known to base station 20 and once received by base station antennas 22-1 to 22-P a channel estimation operation is applied. Consequently, the phase of each and every transmitting antenna 18-1 to 18-N can be derived for the MIMO demodulation. This channel estimation information is then used to derive the best pre-coding weight and feed back to UE 11 in the form of a codebook over the downlink channel (DL). The code book includes phases that are to be applied to the respective transmit antenna so that the multiple signal streams can be de-correlated, and separated cleanly. In addition, the SINR of each signal stream at the base station receiver has to be maintained at certain levels to guarantee the quality of service. In 3G (CDMA/UMTS/HSPA) network, base station feeds back power control bit to maintain the SINR. In addition, the data rate for each data stream could change per TTI (2 ms or 10 ms). Both data rate and power control (i.e., TX power) can be used as the metrics of beamformer performance. In 4G (LTE), Modulation and Coding Scheme (MCS) is fed back to UE to change the transmit data rate for maintaining the service quality.
While the aforementioned mechanism is straightforward to apply in standard MIMO, it would be challenging to achieve in MIMO RDN architecture where there are more transmit antennas (M) than pilot signals (N) in accordance with the 3G-4G standards. Additionally, it would be impossible to determine, when a plurality of UE transmit signals are combined how the phase shift of each of the transmit antennas contributes to the combined pilot signal associated with transmit antennas with a common beamformer. It would be therefore advantageous to find a solution for using existing feedback to UE from the BTS, in order to better adjust the phases of the UE transmit antennas in MIMO RDN configuration.